Presently, electronic memories come in a variety of forms and serve a variety of purposes. For example, one type of memory is flash memory. Generally, flash memories are used for easy and fast information storage in devices such as digital cameras and home video consoles. It is used more as a hard drive than as random access memory (RAM). In fact, flash memory may be considered a solid state storage device (e.g., no moving parts—everything is electronic).
In general, flash memory is a type of electrically erasable programmable read-only memory (EEPROM). It has a grid of columns and rows with a cell that has two transistors at each intersection. The two transistors are separated from each other by a thin tunnel oxide (TOX) layer. One of the transistors is a floating gate, and the other one is a control gate. The floating gate's only link to the row is through the control gate. As long as the link is in place, the cell has a value of one. To change the value to a zero requires a process called Fowler-Nordheim (FN) tunneling.
FN tunneling is used to alter the placement of electrons in the floating gate. For example, an electrical charge is applied to the floating gate and drains to the ground. This charge causes the floating-gate transistor to act similar to an electron gun. That is, the electrons are pushed through and trapped on the other side of the TOX layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A cell sensor then monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, then it has a value of one. However, when the charge passing through the gate drops below the 50 percent threshold, the value changes to zero. Normally, a blank EEPROM has all of the gates fully open, giving each cell a value of one.
The electrons in the cells of a flash-memory can be retuned normal (e.g., one) by the application of an electric field (e.g., a higher voltage charge). Furthermore, flash memory utilizes in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This electrical field erases the target area of the chip, which can then be rewritten. Therefore, flash memory works much faster than traditional EEPROMS because instead of erasing one byte at a time, it erases a block or the entire chip. In addition, flash memory will maintain its data without an external source of power. Thus, it is extremely useful with removable memory media such as digital cameras, digital music players, video consoles, computers, and the like.
However, in order for a flash memory device to operate at peak performance, the TOX layer needs to be as flat as possible. Any variations, such as TOX thickening, TOX thinning, or the like, result in a TOX encroachment issue. That is, the TOX layer may have varying thickness from the center to the edges. This lack of uniformity can result in difficulty during programming or erasing of the memory. One cause of TOX encroachment is the post oxidation process (POP). For example, after an etching process, different defects such as segregated edge defects may be found in the memory device. In order to repair the defects, POP is applied. However, POP may cause TOX encroachment on the channel. Additionally, the amount of time required for POP is not standard. That is, the process varies between devices. Therefore, the amount/effects of TOX encroachment are not easily quantified.
Thus, a need exists for a method and system for detecting tunnel oxide encroachment on a memory device. A further need exists for a method and system for detecting tunnel oxide encroachment on a memory device that can quantify the extent of the encroachment. Yet another need exists for a method and system for detecting tunnel oxide encroachment on a memory device which can be applied during the manufacturing process. A further need exists for a method which meets the above needs and which is compatible with existing memory manufacturing processes.